Assessment Of Diffusion Gradients In Thin Films Technique In Measurement Of The Arsenic Bioavailability In Soils

The Diffusion Gradients in Thin Films (DGT) technique based on Fick's law is an effective method to measure bioavailability of the heavy metals in waters, sediments and soils. As the intensity, quantity and diffusion coefficient of metal elements affect the bioavailability of metal in environment is comprehensively considered, the DGT measurement is payed more and more attentions. There are some evidences that effective concentration of Cu, Zn, Pb, Cd, etc. in soils measured by DGT provide a better indication of metals availability, but the researches on the DGT measurement of bioavailable As in soils are less available. The objectiv of this study was to assess the capability of the DGT technique in predicting the bioavailability of As in soil using pot experiments with 43 soils and to obtain parameters of the kinetics of As transport between soil solution and solid phase using DIFS model. This study is of great significance to provide effective method of bioavailable As measurement in soils and to improve the veracity of risk assessment of soil As. The main results are as follows:1. The simulating lab test were conducted to evaluate the feasibility of using DGT for the measurement of inorganic As under natural conditions. Various factors including pH and As species that may affect the adsorption of the As species to the iron-hydroxide adsorbent were investigated and the maximum and effective As adsorption capacities of iron-hydroxide gel was also considered. There are no significant differences between the concentrations measured by DGT and by direct measurement using hydride generation atomic fluorescence spectrometry (HG-AFS). The results indicated that the maximum and effective As adsorption capacities of iron-hydroxide gel used in this study were limited to≤42μg and≤13μg As, respectively. Under laboratory conditions, pH and As species in solutions were demonstrated not to affect the concentration measured by using DGT.2. Bio-indicator method was introduced to evaluate the dependence of concentrations of As in plant tissue (Edible rape) on the effective concentration (C_E), soil solution As, chemical extracted As and total soil As. The results indicated that plant As were linearly related and highly correlated with C_E, measured by DGT, and the dependent linear equation before and after growth were y = 0.5218x -1.4478 (R² = 0.8401, n = 43) and y = 0.5205x - 1.3863 (R² = 0.8309, n = 43) , respectively. The plant As and soil solution As are closely related, the correlation coefficients (R²) before and after growth were 0.834 and 0.8101, respectively. Measurement of HCl-extractable As (R² =0.6781), NaHCO3-extractable As (R² =0.6781), and AE-As (R² = 0.5697), Al-As (R² = 0.5552), Ca-As (R² = 0.527) were inferior. Plant As was more scattered and nonlinear with respect to Fe-As (R² = 0.1554), O-As (R² = 0.0636), or soil total As (R² = 0.2693). These results demonstrate that the DGT had promise as a quantitative measurement method of the bioavailable As in soils.3. Comparison of correlativity between C_E, and contents of different As form in soils, the results demonstrated that C_E were linearly related and highly correlated with Al-As (R²=0.6484, P<0.01, n=43), AE-As (R²=0.5309, P<0.01, n=43), Ca-As(R²=0.5292, P<0.01, n=43) and Fe-As(R²=0.2205, P<0.01, n=43), but were more scattered and nonlinear with O-As (R²=0.0698, P<0.01, n=43). Correlation analysis of C_E, plant concentration (C_plant) and soil physical and chemical properties indicated that soil pH and total Phosphorus content were positively correlated with C_E and C_plant, but soil OM and C_EC didn't effect the bioavailability of soil As significantly.4. The R values were obtained by DGT measured concentrations (CDGT) and by centrifugal method (C_soln). Three of the tested soils have big R values, bigger than 0.95, which demonstrated that the local lowering of pore water concentrations would be counteracted by a remobilization to the pore waters, and the rate was sufficiently fast. These cases were therefore referred to as the sustained case. The rest tested soils had lower R value, less than 0.95, which demonstrated that there would be some remobilization, but in sufficient to sustain adjacent pore water concentrations. This was referred to as the partial case. Temporal and spatial changes in concentrations of As in solid and solution phases in soils during in situ DGT deployments would be simulated and resupply parameters could be estimated by DIFS modal and experimental data. The results indicated that all three soils with big R value (>0.95) had a short response time (T_c) and a high desorption rate constants (k_b). For these tested soils, except for several soils, all had a long response time (T_c) and a lower desorption rate constants (k_b), which demonstrated that the resupply of solid phase in soils made lower contributes to the bioavailability of soil As. This also indicated that, for As uptake by plants in these particular soils, As supply from solution phase is the dominant controlling process.Above results showed that plant As had better correlation with C_E, measured by DGT than that with As in soil solution, or HCl extractable As, NaHCO3 extractable As and total soil As, It indicated DGT technique can be used to assess the bioavailability of As in soils. Besides, soil R value can be obtained by equilibrium (centrifugal method) and kinetic (DGT) methods. Temporal and spatial changes in concentrations of As in solid and solution phases in soils during in situ DGT deployments would be simulated and resupply parameters could be estimated by DIFS modal and experimental data, including response time (T_c) and desorption rate constants (k_b). These paremetres are helpful to in-depth understand the bioavailability of soil As. It is concluded that the DGT is effective technique in the prediction and assessment of bioavailability As in soils.